1. Field of the Invention
This invention concerns porous materials and methods of manufacture thereof.
2. Description of the Prior Art
Certain filtering and turbine cooling applications have required a multilayer porous material in which the openings therein can be fairly closely controlled both laterally across the width of the layer and also through the entire thickness of the material so that the overall permeability of the material to fluid flow can be precisely controlled.
A prior art approach to satisfying this requirement is described in U.S. Pat. No. 2,857,657 involving cros winding of successive layers of flattened metal wire onto a mandrel and diffusion bonding of the layers together. While more effective than the relatively imprecise method of simply stacking and bonding layers of wire mesh together, certain drawbacks are nonetheless also inherent with this approach:
High cost of the small diameter wires usually used. PA1 Nonuniformity of wire diameter (affects porosity). PA1 Waste selvage that must be trimmed from the ends of each cylinder (often 10 to 20 per cent). PA1 Periodic cross-over points where porosity is nonuniform. PA1 Need for specialized winding machines. PA1 Lengthy calculation procedure needed for each wind pattern. PA1 Compromises in wind pattern due to machine limitations. PA1 Limited sizes of sheets producible with present equipment. PA1 Wind pattern varies slightly in each successive layer as the winding radius increases.
Another disadvantage is encountered in incorporating such materials into complete structures such as turbine parts, as these materials must often be welded and such wound materials have proven difficult to weld and the resulting welds sometimes produce frayed windings and surface subsidence.
Turbine applications also have created a need for a variable porosity material, i.e., a material in which its permeability differs in different regions of the same sheet. While the above-described method can be utilized to produce this characteristic, it is difficult and lacking in allowing a reasonable latitude of design flexibility.
Accordingly, it is an object of the present invention to provide an accurately controlled permeability multilayer material in which the aforementioned limitations of a wound material are minimized or eliminated.